Advanced liquid crystal-based switchable optical devices for light protection applications: principles and strategies

With the development of optical technologies, transparent materials that provide protection from light have received considerable attention from scholars. As important channels for external light, windows play a vital role in the regulation of light in buildings, vehicles, and aircrafts. There is a need for windows with switchable optical properties to prevent or attenuate damage or interference to the human eye and light-sensitive instruments by inappropriate optical radiation. In this context, liquid crystals (LCs), owing to their rich responsiveness and unique optical properties, have been considered among the best candidates for advanced light protection materials. In this review, we provide an overview of advances in research on LC-based methods for protection against light. First, we introduce the characteristics of different light sources and their protection requirements. Second, we introduce several classes of light modulation principles based on liquid crystal materials and demonstrate the feasibility of using them for light protection. In addition, we discuss current light protection strategies based on liquid crystal materials for different applications. Finally, we discuss the problems and shortcomings of current strategies. We propose several suggestions for the development of liquid crystal materials in the field of light protection.


Introduction
Liquid crystal (LC) devices, such as optical modulators, displays and shutters, often utilize dichroic dye guest molecules to control visible light intensity [1][2][3][4]. Typically, dichroic dyes are rod-like molecules with dimensions comparable to the LC molecules. Thus, they tend to align with the LC director N (the average orientation of rod-like LC-molecules [5]) and follow N as it rotates in response to low-voltage electric fields. Since the absorption of dye molecules depends on their orientation with respect to the light propagation direction and the polarization direction, if this light is linearly polarized [1], switching the LC director allows for tuning the relative fractions of light absorbed by the dye molecules and transmitted through the LC cell. Plasmonic analogs of these guest-host LCs, introduced recently [6-9], extend this principle to rod-like plasmonic nanoparticles with typical dimensions in the range of tens of nanometers. Dichroic dyes [1][2][3][4] and plasmonic nanoparticles, such as gold nanorods (GNRs) [6][7][8][9], have their advantages and disadvantages when used with LCs and were previously utilized only separately in recent studies or applications. For example, a large variety of inexpensive commercially available dyes and their (often high) solubility concentration limits make them ideal for designing switchable light absorption in the visible spectral range, but not in the infrared [1]. In contrast, plasmonic GNRs with longitudinal surface plasmon resonance (SPR) peaks in the infrared range allow for tuning peak absorption within the near infrared but rarely in the short-wavelength part of the visible spectrum [6][7][8][9]. Co-dispersions of plasmonic GNRs were developed recently [10] to demonstrate that shortrange or broadband spectral characteristics can be obtained in the switchable plasmonic guest- host composites by mixing nanorods with different aspect ratios and surface treatments within the LC dispersions, though this again was limited in terms of the accessible spectral range.
In this work, we develop a mesostructured LC composite with both molecular dichroic dyes and plasmonic nanoparticles homogeneously distributed within the host medium. We demonstrate that long axes of dye molecules and GNRs can be spontaneously aligned parallel to N, as well as that the rod-like nanoparticles can be also oriented orthogonally to N by defining perpendicular boundary conditions (BCs) on their surfaces. This allows tuning the transmitted light intensity in both the visible and the near infrared spectral ranges, either inphase or out-of-phase. Using different sample geometries, nematic and cholesteric LCs with negative or positive dielectric anisotropy, we show that the LC composite's switching times can be varied from millisecond to seconds and transmission of both unpolarized and polarized light can be controlled depending on the needs of applications such as smart windows, optical shutters, modulators, etc.

Method and design
GNRs were synthesized following a seed mediated method while using surfactants and pH control to tune their aspect ratio [11,12]. To obtain the longitudinal localized SPR peaks at 780 nm, 815 nm and 1010 nm, the synthesized GNRs were designed to have mean lengths and diameters of 77 × 21 nm, 93 × 23 nm, and 104 × 20 nm, respectively, as determined by transmission electron microscopy (TEM) ( Fig. 1(a)-(c)). GNRs with the longitudinal SPR peak at 815 nm were surface-functionalized with thiol-terminated methoxy-poly(ethylene glycol) (mPEG-SH 5kDa, JemKem Technology) [10]. For some GNRs, porous silica shells were grown around the nanoparticles ( Fig. 1(a),(b)), followed by further surfacefunctionalization with dimethyloctadecyl[3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP, ACROS Organics) according to procedures described elsewhere [8][9][10]. The thickness of the silica shell was controlled to maintain the anisotropic shape of the ensuing nanostructures, as needed for orienting these particles in LCs. The extinction spectra of both silica and PEG capped GNRs in isotropic solvents are shown in Fig. 1(d),(e). To disperse different types of synthesized GNRs in various LCs, we used nematic 4-cyano-4′pentylbiphenyl (5CB, Chengzhi Yonghua Display Materials Co., Ltd.) as an intermediate nematic solvent. First, 60 µL of GNRs dispersion in methanol ~0.91 nmol/L was kept in a 0.5 mL centrifuge tube at 90°C for an hour to fully evaporate the solvent. 15 μL of 5CB was then added and kept under sonication for 5 min at 40°C while in the isotropic phase and then vigorously stirred until it cooled down to the nematic phase. The sample was centrifuged at 2000 rpm for 3 min to remove occasional aggregates caused by the nucleation of nematic domains during the phase transition [13], yielding a stable dispersion 0.68 nmol/L of GNRs within 5CB. To obtain dispersions with negative dielectric anisotropy Δε, these 5CB dispersions of GNRs were mixed with a nematic mixture AMLC-0010 (from AlphaMicron Inc.) with Δε = −3.7 and containing dichroic dyes with strong absorption within the visible spectrum. The mixing of 5CB and AMLC-0010 by a ratio of 1:10 (vol. %) yielded a mixture with an estimated Δε≈-2. The cholesteric pitch p of LC mixtures was maintained within p = 10-60 μm using a left-handed chiral dopant ZLI-811 (from Merck) and measured with the Grandjean-Cano wedge cell method [14]. Alternatively, the dispersion of DMOAP capped silica on GNRs (DMOAP-SiO2-GNRs) in 5CB was mixed with the AMLC-0010 at ratio 1:1 (vol. %) yielding a nematic mixture with Δε≈4.6.
We used different configurations of cells with homeotropic (perpendicular) and planar BCs for the LC far-field director N 0 . In the former case, polyimide SE1211 (Nissan Chemical Industries, Ltd.) was spin-coated onto the glass plates with transparent indium tin oxide (ITO) electrodes at 3000 rpm for 60 s and baked at 180°C for 60 min. In the latter case, a thin layer of 1 wt% aqueous polyvinyl alcohol (PVA, Sigma-Aldrich) was spin-coated and kept in an oven at 100°C for 1h and then unidirectionally rubbed. The cells were assembled using 30 μm silica spacers and glued at their edges with UV-curable NOA-63 glue (Norland Products, Inc.); the cell gap thickness was confirmed by using an optical interference method. We used an Olympus BX-51 polarizing optical microscope (POM) equipped with 10 × , 20 × , and 50 × objectives with numerical aperture 0.3−0.9 along with a CCD camera (GS3-U3-28S5C, from Point Grey Research). The light source was a tungsten-halogen lamp. Extinction and transmittance spectra were recorded using a microscope-mounted spectrometer USB2000-FLG (Ocean Optics) with a broadband (400-1200 nm) polarizer inserted into the optical path after the sample. Electric switching properties, including the threshold voltage and response time, were measured utilizing a photodiode and data acquisition card SCC-68 (National Instruments Co.) controlled by a homemade software [6] written in LabVIEW and a Si amplified photodetector PDA100A (Thorlabs Inc.). The response times, including both the rise time upon applying the voltage and the decay time after removing the voltage, were measured as in our previous studies and are described elsewhere [6][7][8][9][10]. Wavelengthselective studies of switching of dye and GNR orientations utilized optical interference filters such as 700 nm long pass filter (Semrock Inc.) and an infrared filter (Olympus). TEM images were obtained using a FEI Tecnai T12 Spirit.

Experiment and Characterization
GNRs treated with PEG promoted tangential BCs on their surfaces, prompting self-alignment of GNRs within the aligned LC parallel to N (Fig. 2(a)), while DMOAP-SiO 2 -GNRs selfaligned perpendicularly to N because of perpendicular BCs (Fig. 2(b)). This self-alignment ( Fig. 2(a),(b)) is driven by the minimization of the surface anchoring energy [6,7]. The corresponding polarization-dependent extinction spectra of PEG-GNR and DMOAP-SiO 2 -GNRs in uniformly aligned 5CB samples are shown in Fig. 2(c),(d). The rod-like dichroic dyes follow the LC molecules and N because of the induced dipole-dipole (anisotropic van der Waals) interactions [1], similar to that between LC molecules themselves [5,14]. Although the mechanisms of alignment of nanometer-sized rod-like dichroic dye molecules and much larger (20 to 100 nm) nanorods are rather different, the orientational self-ordering of both leads to anisotropic, polarization-dependent optical characteristics (Fig. 2(c),(d)). Orientational self-ordering of different types of GNRs and dye molecules can be quantified by determining the scalar order parameter commonly defined as S = 〈3cos 2 θ-1〉/2, where the angled brackets denote sample average values and θ is the angle between the long axis of rodlike dye molecules or nanoparticles and N [14]. By using samples with the uniformly aligned N 0 defined by the rubbing direction N r in planar LC cells, the order parameter S = (A || -A⊥)/(A || + 2A⊥) of both the dichroic dye and GNRs can be experimentally determined based on the peak absorbance values A || and A⊥ at the linear polarization orientation of the probing light P||N 0 and P⊥N 0 , respectively [1,14]. The PEG-GNRs and DMOAP-SiO 2 -GNRs (Figs. 2(c) and 2(d)) exhibit order parameter values of 0.66 and −0.41, respectively, consistent with their average alignment parallel and perpendicular to N 0 . In a similar way (Fig. 2(c)), we determine an order parameter of ≈0.61 for the dye. Dye molecules and GNRs align with respect to the local director also in samples with spatially varying N, such as the uniformly π-twisted configuration shown in (Fig. 2(e)). In this sample, the dichroic dye molecules and PEG-functionalized GNRs follow the helicoidal N in the LC with Δε<0 and p = 60 μm in a planar cell of thickness d≈p/2≈30 μm. Following N, dye molecules and GNRs twist by π around the helical axis perpendicular to cell substrates. This structure of spatial orientations of dye molecules and nanoparticles is inferred from the extinction spectra of the composite LC (Fig. 2(f)). Due to a large pitch, linear polarizations of visible and near-infrared electromagnetic radiation traversing such LC cell closely follow the rotation of N as the Mauguin parameter Δnp/(2λ)>>1 [14], where the optical anisotropy of used mixtures was within Δn≈0.05-0.2 and λ is the wavelength of the incident light. This results in a strong dependence of the extinction spectra on the polarization of the probing light P (Fig. 2(f)) with a maximum extinction due to the dye and the longitudinal SPR of PEG-functionalized GNRs at P||N r . Under the Mauguin regime of light propagation, the strong polarization dependence of the spectra shown in (Fig.  2(f)) confirms that the dye molecules and the rod-like particles mimic the spatial LC director configurations, as depicted in Fig. 2(e).
A cholesteric LC composite with Δε<0, the same as shown in Fig. 2(f), was also introduced into a homeotropic cell with d≈p/2≈30 μm. Despite the cholesteric LC's tendency to twist, strong perpendicular BCs at the substrates of this cell cause unwinding of N (a leftside inset in Fig. 3(a)). Application of 1 kHz AC voltage to the transparent ITO electrodes on the confining substrates forces LC molecules to rotate to be orthogonal to the electric field and tangential to the cell substrates. This realignment is due to the dielectric coupling between N and electric field and leads to the so-called translationally invariant twisted configuration [15] (a right-side inset in Fig. 3(b)) with the threshold voltage 2 2 2 2 2 2 th , where K 22 and K 33 are the twist and bend Frank elastic constants, respectively, and q 0 = 2π/p [16]. The chiral nature of the used cholesteric LC tends to reduce U th of the Freedericksz transition, as compared to the nematic counterpart (for which q 0 = 0), so that it is in the range of several volts despite the small value of Δε. The experimentally measured value of U th ≈5 V (Fig. 3(a)) is consistent with an estimation obtained from the above expression using the sample LC parameters. The extinction spectra ( Fig. 3(b)) exhibit a strong dependence on the applied voltage U>U th , revealing that both GNRs and dye molecules ( Fig. 3(a)-(c)) in this composite follow the changes of the LC orientation with applied U. Interestingly, the PEG-GNR transmission decay faster than the dye's does (Fig. 3(c)).
To characterize the switching kinetics of our LC composite (Fig. 3(d)), we separately probed the response times of both the dichroic dye and the GNRs using optical filters. Rise and decay times were calculated based on the relative light transmittance changes (Fig.  3(e),(f)) between 90% and 10%, respectively. The rise (τrise) and decay (τ decay ) times of switching N are related as  [14]. Mechanical coupling of GNRs and N through the surface anchoring and direct van der Waals interactions between dye and LC molecules previously led to similar relations in guest-host dichroic [1][2][3][4] and plasmonic [6][7][8][9][10] LCs. This is also the case for our guest-host LCs that contain plasmonic GNRs and dichroic dye molecules at the same time ( Fig. 3(d)). Reorientation of dichroic dye and N is faster than that of PEG-functionalized GNRs, which is expected given that the dye molecules are comparable in dimensions to that of LC molecules while GNRs are much larger and exhibit lagging of rotational response, as also observed previously [9]. The average τ decay values for dichroic dye and PEG-functionalized GNRs are 1.33 s and 1.38 s, respectively, relatively long because of the large cell thickness d≈30 μm, whereas τ rise is in the range of subseconds at U>10V (Fig. 3(d)-(f)). These response times can be shortened further by orders of magnitude through reducing d due to the scaling τ decay ∝d 2 , similar to that of pristine LCs [6].
We also designed the nematic LC composite with Δε>0 doped with dichroic dye and DMOAP-SiO 2 -GNRs (Fig. 4). DMOAP imposes homeotropic alignment of LC molecules on the surface of GNR nanostructures, resulting in a novel configuration where GNRs orient in the plane perpendicular to N and are able to freely rotate around N. In a planar cell at U>U th , N reorients to align with the applied field, which is followed by the reorientation of the GNRs and the dye molecules which tend to stay perpendicular and parallel to N, respectively ( Fig.  4(a)). Extinction spectra of this composite demonstrate strong polarization and U dependences ( Fig. 4(b)), allowing for the effective switching between the visible or near infrared absorbance bands in an alternating manner. Using appropriate optical filters, similar values of U th were obtained by measuring independently the optical responses of the GNRs and dye molecules (Fig. 4(c)). The GNR reorientation is slower than both N and dichroic dye molecules' reorientation ( Fig. 4(d)). Characterization of τ rise and τ decay are shown in Fig.  4(e),(f), respectively. The response times in this composite are faster as compared to its counterpart with the negative dielectric anisotropy (Fig. 3(d)), owing to a larger absolute value of dielectric anisotropy of the LC host. Further exploration of cell configurations and LC composites with various additives in the forms of organic and inorganic materials may allow pre-designed switching of electromagnetic radiation in visible and infrared ranges, depending on the needs of applications.

Discussion and conclusions
To conclude, we designed and implemented mesostructured guest-host LC composites based on co-doping nematics and cholesterics with dichroic dyes and shape-anisotropic plasmonic nanoparticles. Our findings demonstrate that nematic and cholesteric LC hosts with positive or negative dielectric anisotropies and different sample geometries can be used to design guest-host composite systems capable of synchronous ( Fig. 3(b)) or alternating (Fig. 4(b)) switching of light transmission in visible and infrared spectral ranges. Relatively short (≤1 s) response times of our guest-host are composites faster than typical responses of solid-state electrochromic devices like the ones utilized in smart windows [17,18]. The response time can be reduced further down to hundreds or even tens of milliseconds by varying gap thickness of the used cells because the LCs are known to respond much faster in thinner cells, as studied before for positive dielectric LCs [6]. Our guest-host mesostructured composites are advantageous compared with other different LC-based composites that are already implemented in smart windows applications. For example, polymer dispersed liquid crystals (PDLC) provide operation modes based on only switching between scattering and clear states. Moreover, the voltage required to switch PDLC is higher by almost one order of magnitude relative to our system, although the response time is faster compared to our system [20,21]. The thermochromic LC systems are driven by higher voltages as well and suffer from nonuniform switching [19]. The dichroic dyes-based conventional guest-host LC displays which show similar response time as our system mainly function in the visible region of the light spectrum, not extending to cover the infrared region of the spectrum [22]. In contrast, we can selectively tune the IR absorbance peak through changing the aspect ratio of the inserted GNRs in these composites to modulate the radiation in many near infrared regions. Although, our examples (Figs. 2(e) and 2(f) and Fig. 3) of switching cholesteric LC configurations exhibit polarization-dependent properties, one can also extend this approach of composite guest-host LCs with both nanoparticles and dyes to the regime of polarization-independent responses when the Mauguin parameter is tuned to be small, Δnp/(2λ)<1, which was previously achieved for guest-host LCs utilizing only dichroic dyes [4]. This would be of interest for applications in a new breed of smart windows.

Funding
This study was partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award ER46921, contract DE-SC0010305 with the University of Colorado Boulder. G.H.S. also acknowledges support from the King Faisal University.